Why is the Ocean Blue ?

The question - Why is the ocean blue ? - is asked often and we have found there is much debate and a few reasons given for why the ocean is mostly some shade of blue.

We give these reasons below.

Personally we think it is blue due to a combination of all these reasons - but we will just give the facts and let you make up your own mind.

The ocean reflects the blue sky

Blue wavelengths are absorbed the least by the deep ocean water and are scattered and reflected back to the observer’s eye

Particles in the water may help to reflect blue light

Most people seem to think the answer to the question, why is the ocean blue is because it reflects the blue sky - after all on a cloudy day the sea looks more gray than blue. Others would say the ocean appears blue because that is the colour our eyes see. But many factors can affect the colour such as the depth of the water, the amount of skylight and particles in the water.

The colour that we see is dependent on the reflection of the visible wavelengths of light to our eyes.

Sunlight is made up of all the colors of the rainbow - red, orange, yellow, green, blue, and violet.

The way wavelengths of light pass through matter differs depending on the composition of the material. Blue wavelengths get transmitted to greater depths of the ocean, with red wavelengths being absorbed quickly. The water molecules scatter the blue wavelengths by absorbing the light waves and then sending the lightwaves back out in different directions. Which explains why our eyes see mostly blue wavelengths and so sees the ocean as mostly blue.

In deep water, most of the sunlight is scattered by oxygen in the water - this scatters more of the blue light, making the water seem less blue.

Though sometimes the ocean looks green. A lot of this is thought to be due to the amount of plant life in the water or the amount of sediment that flows out of rivers into the ocean. More of the blue light gets absorbed and the yellow pigments in the plant life make the blue light that is reflected back appear green.

The water in the ocean can look brown or milky brown after a storm as the winds and currents churn up sand and silt and this can be especially noticeable where the water flows into the ocean at the river mouth.

Some argue that although the ocean can reflect the blue sky, this is only prominent at low angles and the water needs to be fairly smooth and mirror-like.

We hope our explanation to the question - Why is the ocean blue ? has been helpful. If you know of any other reasons that you think should be included here, let us know.

Odd Coloured Seas

The Red Sea lies between Africa and Asia and often looks red because of the red algae that lives in the water.

The Black Sea in Eastern Europe looks black because it has a high concentration of hydrogen sulfide in the water - which appears black.